The present invention relates generally to lash adjuster mechanisms for internal combustion engines and, more particularly, to hydraulic lash adjusters for internal combustion engines.
Lash adjuster mechanisms for valve trains of internal combustion engines are well known in the engine field. Lash adjusters (sometimes referred to as xe2x80x9canti-lash adjustersxe2x80x9d) serve to compensate for thermal expansion and/or wear of valve train components (such as, for example, valve stems) and maintain a constant gap or lash in engine valve actuation mechanisms, while minimizing delays in valve actuation. When engine components are cold (i.e., when the engine is first started after being shut down for an extended period of time), component dimensions are at a relative minimum. For example, in a valve train, component tolerances normally combine to provide a gap or lash of a distance xe2x80x9cX.xe2x80x9d However, when the engine is warmed up shortly after being started, the engine components expand, and due to this thermal expansion, the gap is reduced, for example, by a distance xe2x80x9cTE,xe2x80x9d resulting in a total gap or lash of X minus TE.
In the absence of such a gap or lash X, when the engine is cold, or in the event that TE is greater than X, once the engine warms up, engine valves will be unintentionally opened slightly due to thermal expansion of valve train components. Such unintentional valve opening can cause adverse engine performance, for example, by preventing engine cylinders or combustion chambers from being sealed during the compression stroke or cycle of engine operation.
Hydraulic lash adjusters employ hydraulic fluid, such as, for example, engine lubricating oil, to vary the length of the lash adjuster to compensate for the expansion of valve train components due to thermal effects. Hydraulic lash adjusters may be located at any convenient location within the valve train, such as, for example, within a cam follower, or alternatively, between a cam follower and a push rod.
Automatic lash adjuster mechanisms or xe2x80x9chydraulic liftersxe2x80x9d have been developed that permit the valve train of an internal combustion engine to accommodate thermal expansion and/or wear of valve train components without adverse effects such as unintentional valve opening due to thermal expansion, while at the same time, minimizing, or even eliminating lash and thereby avoiding undesired effects of lash, such as valve opening delays, and valve train component damage and/or wear to valve train components due to impact forces that occur during lash take-up. One example of a hydraulic lash adjuster is disclosed in Krieg, U.S. Pat. No. 4,227,495.
However, conventional hydraulic lash adjuster mechanisms can interfere with the operation of engine compression braking systems. This is because conventional hydraulic lash adjuster mechanisms may respond in an undesirable manner to a gap in the valve train that is created by a brake actuator slightly opening an engine valve. Since the lash adjuster mechanism cannot differentiate between such a gap due to braking and a gap due to valve train component tolerances and/or wear, the lash adjuster mechanism responds by taking up the gap and thereby delaying or preventing the engine valve from closing at a proper time for compression braking. For example, the delay in valve closing may mean that the valve cannot close in time for the next engine braking compression cycle.
The present invention is directed to overcoming one or more of the problems or disadvantages associated with the prior art.
A lash adjuster mechanism includes a first component formed from a first material having a first coefficient of thermal expansion, and a second component formed from a second material having a second coefficient of thermal expansion greater than the first coefficient of thermal expansion. The second component has an exterior dimension that is greater than or equal to a corresponding interior dimension of the first component when the lash adjuster is heated to an operating temperature.
A method of adjusting lash in a mechanism for opening valves in an internal combustion engine is provided. The method includes the steps of providing a first component formed from a first material having a first coefficient of thermal expansion, providing a second component formed from a second material having a second coefficient of thermal expansion greater than the first coefficient of thermal expansion, and heating the first and second components to a temperature sufficient to induce the first and second components to lock up with respect to one another.
The invention provides a method and apparatus for absorbing component tolerances and wear of the valve train when cold, but locking up when hot. One advantage provided by the invention is the avoidance of the lash adjuster causing interference with an engine braking system.